Dissociation of lipopolysaccharide (LPS)-inducible gene expression in murine macrophages pretreated with smooth LPS versus monophosphoryl lipid A

Acyloxyacyl hydrolase deficiency induces chronic inflammation and bone loss in male mice

Hormonal homeostasis is essential in bone remodeling. Recent studies have shown that the treatment of intestinal inflammation can result in the regulation of bone resorption in distant bones. Increased intestinal permeability may lead to systemic inflammation and bone loss, also known as gut-bone axis. However, the underlying mechanism remains to be elucidated. Lipopolysaccharide (LPS) is a component of gram-negative bacteria that can increase osteoclastic differentiation in vitro. Acyloxyacyl hydrolase (AOAH) is a specific degrading enzyme of LPS, but little is known about the role of AOAH in bone metabolism. In this study, adult Aoah^-/- mice showed a chronic inflammatory state and osteopenic phenotype analyzed by micro-CT and HE staining. Tartrate-resistant acid phosphatase (TRAP) staining of femurs showed an increase in TRAP-positive cells from Aoah^-/- mice. AOAH depletion enhanced the osteoclast differentiation and bone resorption capacity of bone marrow-derived macrophages (BMMs). The enhanced osteoclast differentiation and bone resorption capacity of Aoah^-/- BMMs were reversed by rAOAH. In conclusion, the chronic inflammatory state of adult Aoah^-/- mice promotes bone resorption. AOAH participates in bone metabolism, which is mainly mediated by inhibiting osteoclast differentiation. LPS may be a key mediator of the gut-bone axis, and targeting AOAH may represent a feasible strategy for the treatment of chronic inflammatory bone resorption. KEY MESSAGES : AOAH knockout mice exhibited chronic inflammation mediated by LPS, and LPS may also serve as an important mediator in the regulation of bone metabolism in the gut-bone axis. AOAH regulated bone resorption by blocking the osteoclast differentiation via classical ERK and JNK pathways. rAOAH could rescue the enhanced osteoclast differentiation caused by AOAH deficiency.

Dissociation of TRIF bias and adjuvanticity

Toll-like receptors (TLRs), a family of "pattern recognition receptors," bind microbial and host-derived molecules, leading to intracellular signaling and proinflammatory gene expression. TLR4 is unique in that ligand-mediated activation requires the co-receptor myeloid differentiation 2 (MD2) to initiate two signaling cascades: the MyD88-dependent pathway is initiated at the cell membrane, and elicits rapid MAP kinase and NF-κB activation, while the TIR-domain containing adaptor inducing interferon-β (TRIF)-dependent pathway is initiated from TLR4-containing endosomes and results in IRF3 activation. Previous studies associated inflammation with the MyD88 pathway and adjuvanticity with the TRIF pathway. Gram-negative lipopolysaccharide (LPS) is a potent TLR4 agonist, and structurally related molecules signal through TLR4 to differing extents. Herein, we compared monophosphoryl lipid A (sMPL) and E6020, two synthetic, non-toxic LPS lipid A analogs used as vaccine adjuvants, for their capacities to activate TLR4-mediated innate immune responses and to enhance antibody production. In mouse macrophages, high dose sMPL activates MyD88-dependent signaling equivalently to E6020, while E6020 exhibits significantly more activation of the TRIF pathway (a "TRIF bias") than sMPL. Eritoran, a TLR4/MD2 antagonist, competitively inhibited sMPL more strongly than E6020. Despite these differences, sMPL and E6020 adjuvants enhanced antibody responses to comparable extents, with balanced immunoglobulin (Ig) isotypes in two immunization models. These data indicate that a TRIF bias is not necessarily predictive of superior adjuvanticity.

Escherichia coli adhesion portion FimH functions as an adjuvant for cancer immunotherapy

Induction of antigen-specific immune activation by the maturation of dendritic cells (DCs) is a strategy used for cancer immunotherapy. In this study, we find that FimH, which is an Escherichia coli adhesion portion, induces toll-like receptor 4-dependent and myeloid differentiation protein 2-independent DC maturation in mice in vivo. A combined treatment regimen with FimH and antigen promotes antigen-specific immune activation, including proliferation of T cells, production of IFN-γ and TNF-α, and infiltration of effector T cells into tumors, which consequently inhibits tumor growth in mice in vivo against melanoma and carcinoma. In addition, combined therapeutic treatment of anti-PD-L1 antibodies and FimH treatment efficiently inhibits CT26 tumor growth in BALB/c mice. Finally, FimH promotes human peripheral blood DC activation and syngeneic T-cell proliferation and activation. Taken together, these findings demonstrate that FimH can be a useful adjuvant for cancer immunotherapy.

A System Dynamics Model to Predict the Human Monocyte Response to Endotoxins

System dynamics is a powerful tool that allows modeling of complex and highly networked systems such as those found in the human immune system. We have developed a model that reproduces how the exposure of human monocytes to lipopolysaccharides (LPSs) induces an inflammatory state characterized by high production of tumor necrosis factor alpha (TNFα), which is rapidly modulated to enter into a tolerant state, known as endotoxin tolerance (ET). The model contains two subsystems with a total of six states, seven flows, two auxiliary variables, and 14 parameters that interact through six differential and nine algebraic equations. The parameters were estimated and optimized to obtain a model that fits the experimental data obtained from human monocytes treated with various LPS doses. In contrast to publications on other animal models, stimulation of human monocytes with super-low-dose LPSs did not alter the response to a second LPSs challenge, neither inducing ET, nor enhancing the inflammatory response. Moreover, the model confirms the low production of TNFα and increased levels of C-C motif ligand 2 when monocytes exhibit a tolerant state similar to that of patients with sepsis. At present, the model can help us better understand the ET response and might offer new insights on sepsis diagnostics and prognosis by examining the monocyte response to endotoxins in patients with sepsis.

Endotoxin tolerance is not a LPS-specific phenomenon: partial mimicry with IL-1, IL-10 and TGFβ

Administration of non-lethal doses of lipopolysaccharide (LPS) to experimental animals and humans results for a short period of time in a state of hyporesponsiveness to a second LPS challenge. This phenomenon, known as endotoxin tolerance, has been reproduced in vitro using human monocytes, rendered endotoxin-tolerant following a first incubation with LPS. A further activation by LPS was manifested by a dramatically reduced production of tumor necrosis factor α (TNFα). We report this low responsiveness of LPS pretreated monocytes as an endotoxin non-specific phenomenon. Indeed, TNFα release upon further activation with either killed Gram-positive bacteria (Staphylococci, Streptococci) or zymosan was also significantly diminished. This was not the case when phorbol myristate acetate (PMA) was used as a second triggering agent, suggesting that the monocyte hyporesponsiveness due to LPS does not affect all activation pathways, particularly that of protein kinase C. On the other hand, both PMA and zymosan pretreatment could reduce a further activation of monocytes by LPS. We investigated whether the first signal(s) delivered by LPS, could be mimicked by some of the LPS-induced cytokines. Preincubation of monocytes with either interleukin-1 (IL-1), IL-10 or transforming growth factor β (TGFβ) lower the TNFα production upon further activation with LPS. None of the cytokines alone was as efficient as the LPS molecule, but high levels of tolerization were obtained with combination of IL-1, IL-10 and TGFβ. Neither IL-6, IL-8 nor TNFα led to hyporeactive cells. Our data indicate that endotoxin tolerance is not an LPS-specific phenomenon and that more than one cytokine can contribute to render human monocytes hyporeactive to further activation by LPS.

Inhibition of LPS binding on human monocytes by phosphonooxyethyl analogs of lipid A

We investigated the inhibition of LPS binding on human monocytes by synthetic analogs of lipid A. A common characteristic of the analyzed structures is a α-(or β-) phosphonooxyethyl group in position 1 of the GlcN I of the lipid A backbone. Compounds PE-1, PE-2 and PE-3 are analogs of synthetic Escherichia coli lipid A whereas PE-4 represents an analog of tetraacyl precursor Ia (synthetic compound 406). By determining the ability of these preparations to inhibit the binding of FITC-labeled LPS (E. coli 0111:B4) on human monocytes the relationship between their structure and cellular binding affinity was evaluated. The results showed a structure-dependent hierarchy of inhibition capacity. Thus, compound PE-1 inhibited the binding of FITC-LPS only slightly more than PE-2. However, compound PE-3, possessing β-configurated GlcN I, exhibited a drastically decreased inhibition capability. Best inhibition was obtained with compound PE-4. It was furthermore shown by a Lineweaver-Burk plot that the inhibition of LPS binding was due to competition of FITC-LPS and PE-4 for the same binding structure. The synthesis of stable 1-phosphonooxyethyl analogs of precursor Ia with high affinity for LPS receptor structures but lacking cytokine-inducing capacity (like PE-4) may be of relevance for their function as potent antagonists of LPS in therapy of endotoxic shock and sepsis.

Effect of inflammatory and antiinflammatory stimuli on acyloxyacyl hydrolase gene expression and enzymatic activity in murine macrophages

Acyloxyacyl hydrolase (AOAH) is an enzyme found in macrophages and neutrophils that specifically cleaves the acyloxyacyl moieties of lipopolysaccharide (LPS), thus rendering it non-toxic for human cells. In the present study, we demonstrate that LPS augments AOAH mRNA expression (10-20-fold) in murine macrophages. Following LPS treatment (100 ng/m]), AOAH mRNA was induced by 2 h, peaked at 6 h, and was sustained over 72 h. Optimal induction of AOAH mRNA was observed with as little as 0.1 ng/ml LPS. LPS also induced a concomitant increase in AOAH enzymatic activity in cytosolic extracts from murine macrophages and the ability of macrophages to deacylate LPS was not diminished in endotoxin-tolerized macrophages. LPS-stimulated AOAH mRNA expression was cycloheximide sensitive, indicating that de novo protein synthesis is required for AOAH mRNA production. Moreover, AOAH mRNA expression was also induced by IFN-γ. LPS-stimulated mRNA expression was not suppressed by either dexamethasone or IL-10. Finally, intraperitoneal challenge of mice with 25 μg of LPS resulted in increases in AOAH mRNA in both the lung (∼3-fold) and in the liver (∼6-fold). A possible role for LPS-inducible AOAH in the elimination of LPS is discussed.

Bacterial endotoxins and pathogenesis of Gram-negative infections: current status and future direction

100 years after the discovery of a bacterial 'endotoxin', 50 years after the introduction of antibiotics and 25 years after the routine use of intensive care units to support septic shock patients, Gram-negative infections continue to account for significant morbidity and mortality. In the coming decade, basic research on the structure/function of LPS, the cytokine cascade, and receptor-mediated intracellular signalling responses to LPS and cytokines will provide a greater understanding of the molecular, cellular and systemic responses to endotoxin and infection. New therapeutic agents now emerging from research, and better designed clinical trials to assess those agents will contribute to the next significant decline in sepsis- and shock-related morbidity and mortality. This article summarizes the findings of a workshop convened at the National Institutes of Health (NIH) to examine current research on endotoxin and Gram-negative septic shock.

Endotoxin-tolerant mice produce an inhibitor of tumor necrosis factor synthesis

The present study was carried out to investigate a new aspect of early endotoxin tolerance in mice. Evidence was obtained that early endotoxin tolerance is associated with the production of an activity interfering with the synthesis of tumor necrosis factor α (TNFα). LPS-tolerant mice, challenged with lipopolysaccharide (LPS) had lower serum levels of TNF than LPS-treated normal animals. In parallel, whole blood of tolerant mice, treated with LPS ex vivo generated lower levels of TNF than blood of normal animals. Plasma obtained 30 min after challenge with LPS from tolerant mice was inhibitory for TNF synthesis in whole blood of normal mice ex vivo. This seems to be specific for tolerant mice, since plasma prepared from normal, endotoxin-challenged mice was devoid of inhibitory activity for TNF production. The TNF-inhibitor in plasma of endotoxin tolerant mice did not interfere with the cytotoxic activity of murine recombinant TNFα on WEHI cells. Heat treatment of plasma destroyed the activity.

Peritoneal macrophages from endotoxin-tolerant mice produce an inhibitor of tumor necrosis factor α synthesis and protect against endotoxin shock

In the present study it was found that development of early endotoxin-tolerance is associated with the capacity of mouse peritoneal macrophages (MPM) to produce an activity interfering with the synthesis of tumor necrosis factor α. Peritoneal macrophages from LPS-tolerant mice (tMPM), treated with LPS in vitro produced less TNFα, IL-10 and TGFβ than LPS-treated macrophages from normal mice (nMPM). The supernatants of LPS-activated tMPM contained activities which suppressed formation of TNF in nMPM and RAW 264.7 cells as determined by bioassay, ELISA and PCR. Supernatants of nMPM and unstimulated tMPM were devoid of the inhibitory activity. The inhibitor did not interfere with the bioactivity of TNFα in WEHI cells. It also suppressed PMA/IFN-γ induced TNF synthesis in macrophage cultures. The transfer of macrophages isolated from endotoxin-tolerant mice into normal mice protected against endotoxin shock, whereas macrophages from normal mice increased susceptibility to endotoxin.

Low-grade inflammatory polarization of monocytes impairs wound healing

Impaired wound healing often accompanies low-grade inflammatory conditions, during which circulating levels of subclinical super-low-dose endotoxin may persist. Low-grade inflammatory monocyte polarization may occur during chronic inflammation and deter effective wound repair. However, little is understood about the potential mechanisms of monocyte polarization by sustained insult of subclinical super-low-dose endotoxin. We observed that super-low-dose endotoxin preferentially programmes a low-grade inflammatory monocyte state in vitro and in vivo, as represented by the elevated population of CD11b(+) Ly6C(high) monocytes and sustained expression of CCR5. Mechanistically, super-low-dose endotoxin caused cellular stress, altered lysosome function and increased the transcription factor IRF5. TUDCA, a potent inhibitor of cellular stress, effectively blocked monocyte polarization and improved wound healing in mice injected with super-low-dose endotoxin. Our data revealed the polarization of low-grade inflammatory monocytes by sustained endotoxin challenge, its underlying mechanisms and a potential intervention strategy. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

High Fat High Cholesterol Diet (Western Diet) Aggravates Atherosclerosis, Hyperglycemia and Renal Failure in Nephrectomized LDL Receptor Knockout Mice: Role of Intestine Derived Lipopolysaccharide

A high fat meal, frequently known as western diet (WD), exacerbates atherosclerosis and diabetes. Both these diseases are frequently associated with renal failure. Recent studies have shown that lipopolysaccharide (LPS) leaks into the circulation from the intestine in the setting of renal failure and after WD. However, it is not clear how renal function and associated disorders are affected by LPS. This study demonstrates that circulatory LPS exacerbates renal insufficiency, atherosclerosis and glucose intolerance. Renal insufficiency was induced by 2/3 nephrectomy in LDL receptor knockout mice. Nx animals were given normal diet (Nx) or WD (Nx+WD). The controls were sham operated animals on normal diet (control) and WD (WD). To verify if LPS plays a role in exaggerating renal insufficiency, polymyxin (PM), a known LPS antagonist, and curcumin (CU), a compound known to ameliorate chronic kidney disease (CKD), was given to Nx animals on western diet (Nx+WD+PM and Nx+WD+CU, respectively). Compared to control, all other groups displayed increased circulatory LPS. The Nx+WD cohort had the highest levels of LPS. Nx group had significant renal insufficiency and glucose intolerance but not atherosclerosis. WD had intense atherosclerosis and glucose intolerance but it did not show signs of renal insufficiency. Compared to other groups, Nx+WD had significantly higher cytokine expression, macrophage infiltration in the kidney, renal insufficiency, glucose intolerance and atherosclerosis. PM treatment blunted the expression of cytokines, deterioration of renal function and associated disorders, albeit not to the levels of Nx, and was significantly inferior to CU. PM is a non-absorbable antibiotic with LPS binding properties, hence its beneficial effect can only be due to its effect within the GI tract. We conclude that LPS may not cause renal insufficiency but can exaggerate kidney failure and associated disorders following renal insufficiency.

Super-low dose endotoxin pre-conditioning exacerbates sepsis mortality

Sepsis mortality varies dramatically in individuals of variable immune conditions, with poorly defined mechanisms. This phenomenon complements the hypothesis that innate immunity may adopt rudimentary memory, as demonstrated in vitro with endotoxin priming and tolerance in cultured monocytes. However, previous in vivo studies only examined the protective effect of endotoxin tolerance in the context of sepsis. In sharp contrast, we report herein that pre-conditioning with super-low or low dose endotoxin lipopolysaccharide (LPS) cause strikingly opposite survival outcomes. Mice pre-conditioned with super-low dose LPS experienced severe tissue damage, inflammation, increased bacterial load in circulation, and elevated mortality when they were subjected to cecal-ligation and puncture (CLP). This is in contrast to the well-reported protective phenomenon with CLP mice pre-conditioned with low dose LPS. Mechanistically, we demonstrated that super-low and low dose LPS differentially modulate the formation of neutrophil extracellular trap (NET) in neutrophils. Instead of increased ERK activation and NET formation in neutrophils pre-conditioned with low dose LPS, we observed significantly reduced ERK activation and compromised NET generation in neutrophils pre-conditioned with super-low dose LPS. Collectively, our findings reveal a mechanism potentially responsible for the dynamic programming of innate immunity in vivo as it relates to sepsis risks.

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Super-low dose endotoxin pre-conditioning exacerbates, while higher dose endotoxin alleviates sepsis mortality.

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Super-low dose endotoxin reduces, while higher dose endotoxin facilitates neutrophil extracellular trap (NET) formation.

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Super-low dose endotoxin suppresses, while higher dose endotoxin induces ERK activation required for NET formation.

Type I interferon signaling contributes to the bias that Toll-like receptor 4 exhibits for signaling mediated by the adaptor protein TRIF

Signaling by Toll-like receptor 4 (TLR4) is mediated by either of two adaptor proteins: myeloid differentiation marker 88 (MyD88) or Toll-interleukin-1 (IL-1) receptor (TIR) domain-containing adaptor inducing interferon-β (TRIF). Whereas MyD88-mediated signaling leads to proinflammatory responses, TRIF-mediated signaling leads to less toxic immunostimulatory responses that are beneficial in boosting vaccine responses. The hypothesis that monophosphorylated lipid A structures act as TRIF-biased agonists of TLR4 offered a potential mechanism to explain their clinical value as vaccine adjuvants, but studies of TRIF-biased agonists have been contradictory. In experiments with mouse dendritic cells, we found that irrespective of the agonist used, TLR4 functioned as a TRIF-biased signaling system through a mechanism that depended on the autocrine and paracrine effects of type I interferons. The TLR4 agonist synthetic lipid A induced expression of TRIF-dependent genes at lower concentrations than were necessary to induce the expression of genes that depend on MyD88-mediated signaling. Blockade of type I interferon signaling selectively decreased the potency of lipid A (increased the concentration required) in inducing the expression of TRIF-dependent genes, thereby eliminating adaptor bias. These data may explain how high-potency TLR4 agonists can act as clinically useful vaccine adjuvants by selectively activating TRIF-dependent signaling events required for immunostimulation, without or only weakly activating potentially harmful MyD88-dependent inflammatory responses.

Reprogramming of murine macrophages through TLR2 confers viral resistance via TRAF3-mediated, enhanced interferon production

The cell surface/endosomal Toll-like Receptors (TLRs) are instrumental in initiating immune responses to both bacteria and viruses. With the exception of TLR2, all TLRs and cytosolic RIG-I-like receptors (RLRs) with known virus-derived ligands induce type I interferons (IFNs) in macrophages or dendritic cells. Herein, we report that prior ligation of TLR2, an event previously shown to induce “homo” or “hetero” tolerance, strongly “primes” macrophages for increased Type I IFN production in response to subsequent TLR/RLR signaling. This occurs by increasing activation of the transcription factor, IFN Regulatory Factor-3 (IRF-3) that, in turn, leads to enhanced induction of IFN-β, while expression of other pro-inflammatory genes are suppressed (tolerized). In vitro or in vivo “priming” of murine macrophages with TLR2 ligands increase virus-mediated IFN induction and resistance to infection. This priming effect of TLR2 is mediated by the selective upregulation of the K63 ubiquitin ligase, TRAF3. Thus, we provide a mechanistic explanation for the observed antiviral actions of MyD88-dependent TLR2 and further define the role of TRAF3 in viral innate immunity.

In response to viral infection, cells of the innate immune system synthesize and release members of the type I interferon protein family. The interferons form an essential line of defense, both by slowing viral growth and by expanding the cellular immune response. The synthesis of interferon is initiated by recognition of viral constituents by one or more innate receptors. Among these receptors, Toll like receptor 2 (TLR2) has been shown to be critical for the immune response to a number of viruses, yet TLR2 only directly initiates Type I interferon production in a very small set of innate immune cells. We have discovered that TLR 2 can contribute to the antiviral interferon response much more broadly by indirectly governing the production of interferon induced by other Toll like receptors as wells as downstream of the cytosolic Rig-I like receptors. This happens through the TLR2-dependent up-regulation of a critical signaling element, TRAF3. We also demonstrate that this TLR2 dependent regulation of interferon may be important in biological scenarios involving co-infection of virus and Gram positive bacteria, but not Gram negative bacteria.

Unlipidated outer membrane protein Omp16 (U-Omp16) from Brucella spp. as nasal adjuvant induces a Th1 immune response and modulates the Th2 allergic response to cow's milk proteins

The discovery of novel mucosal adjuvants will help to develop new formulations to control infectious and allergic diseases. In this work we demonstrate that U-Omp16 from Brucella spp. delivered by the nasal route (i.n.) induced an inflammatory immune response in bronchoalveolar lavage (BAL) and lung tissues. Nasal co-administration of U-Omp16 with the model antigen (Ag) ovalbumin (OVA) increased the amount of Ag in lung tissues and induced OVA-specific systemic IgG and T helper (Th) 1 immune responses. The usefulness of U-Omp16 was also assessed in a mouse model of food allergy. U-Omp16 i.n. administration during sensitization ameliorated the hypersensitivity responses of sensitized mice upon oral exposure to Cow’s Milk Protein (CMP), decreased clinical signs, reduced anti-CMP IgE serum antibodies and modulated the Th2 response in favor of Th1 immunity. Thus, U-Omp16 could be used as a broad Th1 mucosal adjuvant for different Ag formulations.

Persistently active microbial molecules prolong innate immune tolerance in vivo

Measures that bolster the resolution phase of infectious diseases may offer new opportunities for improving outcome. Here we show that inactivation of microbial lipopolysaccharides (LPS) can be required for animals to recover from the innate immune tolerance that follows exposure to Gram-negative bacteria. When wildtype mice are exposed to small parenteral doses of LPS or Gram-negative bacteria, their macrophages become reprogrammed (tolerant) for a few days before they resume normal function. Mice that are unable to inactivate LPS, in contrast, remain tolerant for several months; during this time they respond sluggishly to Gram-negative bacterial challenge, with high mortality. We show here that prolonged macrophage reprogramming is maintained in vivo by the persistence of stimulatory LPS molecules within the cells' in vivo environment, where naïve cells can acquire LPS via cell-cell contact or from the extracellular fluid. The findings provide strong evidence that inactivation of a stimulatory microbial molecule can be required for animals to regain immune homeostasis following parenteral exposure to bacteria. Measures that disable microbial molecules might enhance resolution of tissue inflammation and help restore innate defenses in individuals recovering from many different infectious diseases.

We showed previously that mice lacking acyloxyacyl hydrolase (AOAH), the host enzyme that inactivates Gram-negative bacterial lipopolysaccharides (LPS), are unable to regain normal immune responsiveness for many weeks/months after they are exposed in vivo to a small amount of LPS or Gram-negative bacteria. The many possible explanations for slow recovery included long-lasting epigenetic changes in macrophages or other host cells, chronically stimulated cells that produce certain mediators, and persistent signaling by internalized LPS within macrophages. Using several in vivo techniques to study peritoneal macrophages, we found that none of these mechanisms was correct. Rather, prolonged recovery is caused by intact LPS that remains in the environment where macrophages live and can pass from one cell to another in vivo. This is the first evidence that the persistence of a bioactive microbial agonist, per se, can prevent resolution of inflammation in vivo. It also identifies the stimulatory microbial molecule as a realistic target for intervention – in further support, we found that providing recombinant AOAH can be partially preventive. In a larger sense, showing that chemical inactivation of one important microbial signaling molecule is required for full recovery should encourage efforts to find out whether disabling other microbial agonists (chitin, lipopeptides, flagella, others) also benefits infected animals.

Molecular mechanism responsible for the priming of macrophage activation

Host macrophages can be preprogrammed into opposing primed or tolerant states depending upon the nature and quantities of external stimulants. The paradigm of priming and tolerance has significant implications in the pathogenesis and resolution of both acute and chronic inflammatory diseases. However, the responsible mechanisms are not well understood. Here, we report that super low dose bacterial endotoxin lipopolysaccharide (LPS), as low as 5 pg/ml, primes the expression of proinflammatory mediators in macrophages upon a second high dose LPS challenge (100 ng/ml), although 5 pg/ml LPS itself does not trigger noticeable macrophage activation. Mice primed with super low dose LPS (0.5 μg/kg body weight) in vivo experience significantly elevated mortality following a second hit of high dose LPS as compared with saline-primed control mice. Mechanistically, we demonstrate that LPS primes macrophages by removing transcriptional suppressive RelB through interleukin receptor-associated kinase 1 and Tollip (Toll-interacting protein)-dependent mechanisms. This is in sharp contrast to the well documented RelB stabilization and induction by high dose LPS, potentially through the phosphoinositide 3-kinase (PI3K) pathway. Super low dose and high dose LPS cause opposing modulation of interleukin receptor-associated kinase 1 and PI3K pathways and lead to opposing regulation of RelB. The pathway switching induced by super low versus high dose LPS underscores the importance of competing intracellular circuitry during the establishment of macrophage priming and tolerance.

Sustained generation of nitric oxide and control of mycobacterial infection requires argininosuccinate synthase 1

Nitric oxide (NO) defends against intracellular pathogens, but its synthesis must be regulated due to cell and tissue toxicity. During infection, macrophages import extracellular arginine to synthesize NO, generating the byproduct citrulline. Accumulated intracellular citrulline is thought to fuel arginine synthesis catalyzed by argininosuccinate synthase (Ass1) and argininosuccinate lyase (Asl), which would lead to abundant NO production. Instead, we find that citrulline is exported from macrophages during early stages of NO production with <2% retained for recycling via the Ass1-Asl pathway. Later, extracellular arginine is depleted, and Ass1 expression allows macrophages to synthesize arginine from imported citrulline to sustain NO output. Ass1-deficient macrophages fail to salvage citrulline in arginine-scarce conditions, leading to their inability to control mycobacteria infection. Thus, extracellular arginine fuels rapid NO production in activated macrophages, and citrulline recycling via Ass1 and Asl is a fail-safe system that sustains optimum NO production.

A strategy to study pathway cross-talks of cells under repetitive exposure to stimuli

Background

Cells are subject to fluctuating and multiple stimuli in their natural environment. The signaling pathways often crosstalk to each other and give rise to complex nonlinear dynamics. Specifically repetitive exposure of a cell to a same stimulus sometime leads to augmented cellular responses. Examples are amplified proinflammatory responses of innate immune cells pretreated with a sub-threshold then a high dose of endotoxin or cytokine stimulation. This phenomenon, called priming effect in the literature, has important pathological and clinical significances.

Results

In a previous study, we enumerated possible mechanisms for priming using a three-node network model. The analysis uncovered three mechanisms. Based on the results, in this work we developed a straightforward procedure to identify molecular candidates contributing to the priming effect and the corresponding mechanisms. The procedure involves time course measurements, e.g., gene expression levels, or protein activities under low, high, and low + high dose of stimulant, then computational analysis of the dynamics patterns, and identification of functional roles in the context of the regulatory network. We applied the procedure to a set of published microarray data on interferon-γ-mediated priming effect of human macrophages. The analysis identified a number of network motifs possibly contributing to Interferon-γ priming. A further detailed mathematical model analysis further reveals how combination of different mechanisms leads to the priming effect.

Conclusions

One may perform systematic screening using the proposed procedure combining with high throughput measurements, at both transcriptome and proteome levels. It is applicable to various priming phenomena.

Molecular mechanisms responsible for the selective and low-grade induction of proinflammatory mediators in murine macrophages by lipopolysaccharide

Low-dose endotoxemia is prevalent in humans with adverse health conditions, and it correlates with the pathogenesis of chronic inflammatory diseases such as atherosclerosis, diabetes, and neurologic inflammation. However, the underlying molecular mechanisms are poorly understood. In this study, we demonstrate that subclinical low-dose LPS skews macrophages into a mild proinflammatory state, through cell surface TLR4, IL-1R-associated kinase-1, and the Toll-interacting protein. Unlike high-dose LPS, low-dose LPS does not induce robust activation of NF-κB, MAPKs, PI3K, or anti-inflammatory mediators. Instead, low-dose LPS induces activating transcription factor 2 through Toll-interacting protein-mediated generation of mitochondrial reactive oxygen species, allowing mild induction of proinflammatory mediators. Low-dose LPS also suppresses PI3K and related negative regulators of inflammatory genes. Our data reveal novel mechanisms responsible for skewed and persistent low-grade inflammation, a cardinal feature of chronic inflammatory diseases.

Synthetic mimics of antimicrobial peptides with immunomodulatory responses

A new series of aryl-based synthetic mimics of antimicrobial peptides (SMAMPs) with antimicrobial activity and selectivity have been developed via systematic tuning of the aromatic groups and charge. The addition of a pendant aromatic group improved the antimicrobial activity against Gram-negative bacteria, while the addition of charge improved the selectivity. SMAMP 4 with six charges and a naphthalene central ring demonstrated a selectivity of 200 against both Staphylococcus aureus and Escherichia coli , compared with a selectivity of 8 for the peptide MSI-78. In addition to the direct antimicrobial activity, SMAMP 4 exhibited specific immunomodulatory activities in macrophages both in the presence and in the absence of lipopolysaccharide, a TLR agonist. SMAMP 4 also induced the production of a neutrophil chemoattractant, murine KC, in mouse primary cells. This is the first nonpeptidic SMAMP demonstrating both good antimicrobial and immunomodulatory activities.

Network topologies and dynamics leading to endotoxin tolerance and priming in innate immune cells

The innate immune system, acting as the first line of host defense, senses and adapts to foreign challenges through complex intracellular and intercellular signaling networks. Endotoxin tolerance and priming elicited by macrophages are classic examples of the complex adaptation of innate immune cells. Upon repetitive exposures to different doses of bacterial endotoxin (lipopolysaccharide) or other stimulants, macrophages show either suppressed or augmented inflammatory responses compared to a single exposure to the stimulant. Endotoxin tolerance and priming are critically involved in both immune homeostasis and the pathogenesis of diverse inflammatory diseases. However, the underlying molecular mechanisms are not well understood. By means of a computational search through the parameter space of a coarse-grained three-node network with a two-stage Metropolis sampling approach, we enumerated all the network topologies that can generate priming or tolerance. We discovered three major mechanisms for priming (pathway synergy, suppressor deactivation, activator induction) and one for tolerance (inhibitor persistence). These results not only explain existing experimental observations, but also reveal intriguing test scenarios for future experimental studies to clarify mechanisms of endotoxin priming and tolerance.

Epigenetics, bioenergetics, and microRNA coordinate gene-specific reprogramming during acute systemic inflammation

Acute systemic inflammation from infectious and noninfectious etiologies has stereotypic features that progress through an initiation (proinflammatory) phase, an adaptive (anti-inflammatory) phase, and a resolution (restoration of homeostasis) phase. These phase-shifts are accompanied by profound and predictable changes in gene expression and metabolism. Here, we review the emerging concept that the temporal phases of acute systemic inflammation are controlled by an integrated bioenergy and epigenetic bridge that guides the timing of transcriptional and post-transcriptional processes of specific gene sets. This unifying connection depends, at least in part, on redox sensor NAD(+)-dependent deacetylase, Sirt1, and a NF-κB-dependent p65 and RelB feed-forward and gene-specific pathway that generates silent facultative heterochromatin and active euchromatin. An additional level of regulation for gene-specific reprogramming is generated by differential expression of miRNA that directly and indirectly disrupts translation of inflammatory genes. These molecular reprogramming circuits generate a dynamic chromatin landscape that temporally defines the course of acute inflammation.

Pathogen-associated molecular patterns on biomaterials: a paradigm for engineering new vaccines

Vaccine development has progressed significantly and has moved from whole microorganisms to subunit vaccines that contain only their antigenic proteins. Subunit vaccines are often less immunogenic than whole pathogens; therefore, adjuvants must amplify the immune response, ideally establishing both innate and adaptive immunity. Incorporation of antigens into biomaterials, such as liposomes and polymers, can achieve a desired vaccine response. The physical properties of these platforms can be easily manipulated, thus allowing for controlled delivery of immunostimulatory factors and presentation of pathogen-associated molecular patterns (PAMPs) that are targeted to specific immune cells. Targeting antigen to immune cells via PAMP-modified biomaterials is a new strategy to control the subsequent development of immunity and, in turn, effective vaccination. Here, we review the recent advances in both immunology and biomaterial engineering that have brought particulate-based vaccines to reality.

Immunological stimuli change expression of genes and neutrophil function in fathead minnow Pimephales promelas Rafinesque

Fathead minnows Pimephales promelas were exposed to lipopolysaccharide (LPS) and polyinosinic-polycytidylic acid [poly(I:C)] to observe immunological responses during simulated bacterial and viral challenge at the level of gene expression and granulocyte function. Complementary DNA libraries were created from LPS- and poly(I:C)-treated fish and c. 5000 expressed sequence tags (ESTs) were sequenced. The ESTs were subjected to BLASTx analysis and 1500 genes were annotated, grouped by function and 20 immune genes were selected for expression studies by real-time PCR. Lipopolysaccharide treatment significantly downregulated expression of interferon regulatory factor 2 binding protein 1 (nine-fold), Chemokine (C-X-C motif) ligand 12a (three-fold) and TNF-related apoptosis-inducing ligand, TRAIL (two-fold). In poly(I:C)-treated fish, a significant upregulation was observed for IFN-inducible and antiviral proteins belonging to the family of Mx proteins (73-fold) and chemokine CCL-C5a (28-fold). Blood neutrophil count was significantly increased in poly(I:C)-treated fish at 24 and 48 h post-injection. Neutrophil extracellular trap release and respiratory burst of kidney granulocytes were suppressed in poly(I:C)-treated fish, while degranulation of primary granules was not affected significantly by the treatment. The changes in gene expression and neutrophil function in P. promelas exposed to LPS and poly(I:C) support the use of this species as an alternative model for studies of pathogen effects on the innate immune system of fishes.

Generation and characterisation of therapeutic tolerogenic dendritic cells for rheumatoid arthritis

OBJECTIVES

Tolerogenic dendritic cells (tolDCs) constitute a promising experimental treatment for targeting autoreactive T cells in autoimmune diseases, including rheumatoid arthritis (RA). The authors' goal is to bring tolDC therapy for RA to the clinic. Here the authors address key translational issues related to the manufacturing of tolDCs from RA patients with current good manufacturing practice (cGMP)-compliant reagents, the stability of tolDCs, and the selection of suitable quality control markers.

METHODS

Human monocyte-derived tolDCs were established from RA patients and healthy controls (HCs) using the immunosuppressive drugs dexamethasone and vitamin D₃, and the cGMP-grade immunomodulator, monophosphoryl lipid A, in the cGMP-compliant medium, CellGroDC. The functionality of tolDCs and tolDC-modulated autologous CD4 T cells was determined by flow cytometry, [³H]thymidine incorporation and ELISA.

RESULTS

Clinical-grade tolDCs established from patients with RA exhibit a typical tolerogenic phenotype of reduced costimulatory molecules, low production of proinflammatory cytokines and impaired stimulation of autologous antigen-specific T cells, comparable to HC tolDCs. Toll-like receptor 2 (TLR-2) was highly expressed by tolDCs but not mature DCs. Furthermore, tolDCs suppressed mature DC-induced T cell proliferation, interferon γ and interleukin 17 production, and rendered T cells hyporesponsive to further stimulation. Importantly, tolDCs were phenotypically stable in the absence of immunosuppressive drugs and were refractory to further challenge with proinflammatory mediators.

CONCLUSIONS

tolDCs established from patients with RA are comparable to those derived from healthy donors. TLR-2 was identified as an ideal marker for quality control of tolDCs. Potently tolerogenic and highly stable, these tolDCs are a promising cellular therapeutic for tailored immunomodulation in the treatment of RA.

A Toll-like receptor-responsive kinase, protein kinase R, is inactivated in endotoxin tolerance through differential K63/K48 ubiquitination

Overwhelming inflammation triggered by systemic infection in bacterial sepsis contributes to the pathology of this condition. Toll-like receptors (TLRs) are important in early septic inflammation. As a safeguard, the innate immune system has evolved to counter excessive inflammation through the induction of “tolerance.” In endotoxin tolerance, TLR signaling is inhibited and/or attenuated by multiple mechanisms that mitigate the ability of lipopolysaccharide (LPS) to activate critical kinases through TLR4. Here, we describe a novel mechanism. Protein kinase R (PKR), a kinase normally activated by a subset of TLRs, is rendered unresponsive to LPS in endotoxin-tolerized cells. In its naive state, PKR is subject to K63-linked ubiquitination (Ub), followed by K48-linked Ub, in response to LPS. In tolerance, the kinetics of this differential Ub is altered, resulting in a predominance of K48-linked chains, concomitant with a loss of PKR activation. These findings provide a novel mechanism by which a TLR-responsive kinase may be rendered inactive in tolerance.

“Endotoxin tolerance” is a period of transient unresponsiveness to the lipopolysaccharide (LPS) outer membrane component of Gram-negative bacteria that is induced by prior exposure to LPS through Toll-like receptor 4 (TLR4). The loss of LPS-inducible cytokine production by macrophages from patients who have experienced Gram-negative sepsis is well documented, and the increased susceptibility of such patients to reinfection has been attributed to the development of endotoxin tolerance. Multiple mechanisms have been proffered to account for this attenuated response. Using the LPS-responsive kinase protein kinase R (PKR), we have identified differential K48 versus K63 ubiquitination as an additional molecular mechanism by which signal-transducing elements may be inactivated in a state of endotoxin tolerance. This work is highly significant because it links recent discoveries concerning the important role of ubiquitination of signaling molecules in regulating TLR signaling with the loss of LPS responsiveness in tolerance.

Mycoplasma suppression of THP-1 Cell TLR responses is corrected with antibiotics

Mycoplasma contamination of cultured cell lines is a serious problem in research, altering cellular response to different stimuli thus compromising experimental results. We found that chronic mycoplasma contamination of THP-1 cells suppresses responses of THP-1 cells to TLR stimuli. For example, E. coli LPS induced IL-1 β was suppressed by 6 fold and IL-8 by 10 fold in mycoplasma positive THP-1 cells. Responses to live F. novicida challenge were suppressed by 50-fold and 40-fold respectively for IL-1β and IL-8. Basal TLR4 expression level in THP-1 cells was decreased by mycoplasma by 2.4-fold (p = 0.0003). Importantly, cell responses to pathogen associated molecular patterns are completely restored by mycoplasma clearance with Plasmocin. Thus, routine screening of cell lines for mycoplasma is important for the maintenance of reliable experimental data and contaminated cell lines can be restored to their baseline function with antibiotic clearance of mycoplasma.

Signaling crosstalk during sequential TLR4 and TLR9 activation amplifies the inflammatory response of mouse macrophages

The TLR family of pattern recognition receptors is largely responsible for meditating the activation of macrophages by pathogens. Because macrophages may encounter multiple TLR ligands during an infection, signaling crosstalk between TLR pathways is likely to be important for the tailoring of inflammatory reactions to pathogens. Here, we show that rather than inducing tolerance, LPS pretreatment primed the inflammatory response (e.g., TNF production) of mouse bone marrow-derived macrophages (BMM) to the TLR9 ligand, CpG DNA. The priming effects of LPS, which correlated with enhanced Erk1/2, JNK, and p38 MAPK activation, appeared to be mediated via both c-Fms-dependent and -independent mechanisms. LPS pretreatment and inhibition of the M-CSF receptor, c-Fms, with GW2580 had comparable effects on CpG DNA-induced Erk1/2 and p38 MAPK activation. However, c-Fms inhibition did not enhance CpG DNA-induced JNK activation; also, the levels of TNF produced were significantly lower than those from LPS-primed BMM. Thus, the priming effects of LPS on TLR9 responses appear to be largely mediated via the c-Fms-independent potentiation of JNK activity. Indeed, inhibition of JNK abrogated the enhanced production of TNF by LPS-pretreated BMM. The c-Fms-dependent priming effects of LPS are unlikely to be a consequence of the inhibitory constraints of M-CSF signaling on TLR9 expression being relieved by LPS; instead, LPS may exert its priming effects via signaling molecules downstream of TLR9. In summary, our findings highlight the importance of signaling crosstalk between TLRs, as well as between TLRs and c-Fms, in regulating the inflammatory reaction to pathogens.

Lipid A-mediated tolerance and cancer therapy

The term "tolerance" from an immunological perspective, broadly encompasses a number of phenomena, but generally refers to a diminished responsiveness to LPS and/or other microbial products. With the discovery that many of the immunological, physiological and/or pathophysiological effects of LPS can be attributed to the lipid A moiety of the LPS molecule, a number of different lipid A analogs were synthesized with the goal of developing a drug that could be used clinically to treat cancer. In many instances, the development of tolerance to the lipid A congeners confounded the utility of these analogs as cancer therapeutics. In certain circumstances, however, the development of tolerance in patients has been utilized therapeutically to protect immunosuppressed patients from sepsis. Although numerous studies have been designed to investigate the development of tolerance, the underlying molecular mechanism remains unclear. This may be due, in part, to differences in the experimental models used, the sources and types of microbes and microbial products studied, kinetics of responses, and/or other experimental conditions. Nonetheless, a number of different signaling pathways have been identified as potentially modulating and/or triggering the development of tolerance. Though complex and incompletely understood, the capacity of tolerance to impact lipid A-based therapeutics, either positively or negatively, is inarguable, thus underscoring the necessity for further investigation toward elucidating the mechanisms contributing to the development of tolerance to lipid A and its analogs.

Host inactivation of bacterial lipopolysaccharide prevents prolonged tolerance following gram-negative bacterial infection

A transient state of tolerance to microbial molecules accompanies many infectious diseases. Such tolerance is thought to minimize inflammation-induced injury, but it may also alter host defenses. Here we report that recovery from the tolerant state induced by Gram-negative bacteria is greatly delayed in mice that lack acyloxyacyl hydrolase (AOAH), a lipase that partially deacylates the bacterial cell-wall lipopolysaccharide (LPS). Whereas wild-type mice regained normal responsiveness within 14 days after they received an intraperitoneal injection of LPS or Gram-negative bacteria, AOAH-deficient mice had greatly reduced proinflammatory responses to a second LPS injection for at least 3 weeks. In contrast, LPS-primed Aoah- knockout mice maintained an anti-inflammatory response, evident from their plasma levels of interleukin-10 (IL-10). LPS-primed Aoah-knockout mice experiencing prolonged tolerance were highly susceptible to virulent E. coli challenge. Inactivating LPS, an immunostimulatory microbial molecule, is thus important for restoring effective host defenses following Gram-negative bacterial infection in animals.

Clinical investigations of Toll-like receptor agonists

BACKGROUND

Toll-like receptors (TLR) represent a family of surface molecules that function as primary sensors of the innate immune system to recognize microbial pathogens. Ligand binding to TLR results in activation of cellular signaling pathways that regulate expression of genes involved in inflammation and immunity.

OBJECTIVE

Use of synthetic TLR ligands (agonists) for treatment and prevention of infectious and neoplastic diseases.

METHODS

Review of literature about clinical investigations of agonists of TLR 4, 7, 8, and 9.

RESULTS/CONCLUSIONS

Imiquimod was the first TLR agonist approved for treatment of anogenital warts, actinic keratosis and superficial basal cell carcinoma in humans. Several other agonists of TLRs 4, 7, 8 and 9 were also shown to be effective for treatment of infections and cancers and, furthermore, were used as adjuvants for vaccination. Based on safety and efficacy of the TLR agonists used to date, applications are likely to increase in the future.

Supplementation of whole-cell pertussis vaccines with lipopolysaccharide analogs: modification of vaccine-induced immune responses

Lipopolysaccharide (LPS) is one of the main constituents of the Gram-negative bacterial outer membrane. Besides being an endotoxin, LPS also possesses a powerful adjuvant activity. Previously, it has been shown that changes in the chemical composition of the lipid A domain of LPS modulate its biological activity. For example, monophosphoryl lipid A (MPL) has been shown to be a non-toxic immunostimulatory compound. Moreover, several LPS analogs have been shown to antagonise LPS-induced signalling in eukaryotic cells. In the present study, we show that supplementation of a whole-cell pertussis (wP) vaccine with LPS analogs modulates the vaccine-induced immune responses. We show in a mouse-model system that addition of MPL to a wP vaccine increases vaccine efficacy without altering vaccine-induced serum pro-inflammatory cytokine levels. Furthermore, we show that Neisseria meningitidis LpxL2 LPS, an LPS species derived from a N. meningitidis lpxL2 mutant, antagonises wP and LPS-stimulated interleukin-6 (IL-6) production by macrophages in vitro, and that addition of this LPS-derivative to the wP vaccine decreases vaccine-induced serum IL-6 levels and increases vaccine efficacy.

Effect of adjuvants and route of immunizations on the immune response to recombinant plague antigens

In this study, we compare four different adjuvants, LT(R192G), CpG ODN, MPL((R))TDM, and alum, for their ability to affect the magnitude, distribution, and duration of antibody responses against F1-V, the lead-candidate antigen for the next generation vaccine against plague, in a murine model. In addition, three different routes of immunization-intranasal (IN), transcutaneous (TC), and subcutaneous (SC) were compared with each adjuvant. Since aerosol exposure to biological warfare agents is of primary concern, both serum and bronchioalveolar lavage (BAL) were analyzed for antigen-specific antibody responses. The most significant findings of the study reported here are that (1) the adjuvant influences the Type 1/Type 2 balance of the antibody response in both the serum and BAL, (2) mucosal immunization is not necessary to obtain F1-V-specific BAL responses, (3) non-traditional adjuvants such as LT(R192G) work when delivered subcutaneously, (4) the route of immunization affects the magnitude of the immune response, and (5) F1-V is highly immunogenic by some routes even in the absence of an exogenously applied adjuvant. These studies provide important insights into the influence of different classes of adjuvants on the immune outcome in biodefense vaccines and for development of new-generation vaccines against other pathogens as well.

Gene-specific control of inflammation by TLR-induced chromatin modifications

Toll-like receptors (TLRs) induce a multi-component inflammatory response that must be tightly regulated to avoid tissue damage. Most known regulatory mechanisms target TLR signalling pathways and thus broadly inhibit multiple aspects of the inflammatory response. Given the functional diversity of TLR-induced genes, we proposed that additional, gene-specific regulatory mechanisms exist to allow individual aspects of the TLR-induced response to be differentially regulated. Using an in vitro system of lipopolysaccharide tolerance in murine macrophages, we show that TLR-induced genes fall into two categories on the basis of their functions and regulatory requirements. We demonstrate that representatives from the two classes acquire distinct patterns of TLR-induced chromatin modifications. These gene-specific chromatin modifications are associated with transient silencing of one class of genes, which includes pro-inflammatory mediators, and priming of the second class, which includes antimicrobial effectors. These findings illustrate an adaptive response in macrophages and reveal component-specific regulation of inflammation.

Toll-like receptor interactions: tolerance of MyD88-dependent cytokines but enhancement of MyD88-independent interferon-beta production

Toll-like receptors (TLRs) signal through two main pathways: a myeloid differentiation factor (MyD)88-dependent pathway that acts via nuclear factor kappaB (NF-kappaB) to induce proinflammatory cytokines such as tumour necrosis factor-alpha (TNF-alpha) and a MyD88-independent pathway that acts via type I interferons to increase the expression of interferon-inducible genes. Repeated signalling through TLR4 and a number of other TLRs has been reported to result in a reduction in the subsequent proinflammatory cytokine response, a phenomenon known as TLR tolerance. In this study we have shown that, whilst NF-kappaB activation and production of TNF-alpha and interleukin-12 by murine RAW264.7 and J774.2 cells in response to stimulation by TLR4, -5, -7 or -9, was reduced by prior stimulation with TLR4, -5, -7 or -9 ligands, the primary stimulation of TLR3, which does not use the MyD88 pathway, did not reduce the TNF-alpha or interleukin-12 responses to subsequent TLR stimulation. The response to TLR3 stimulation was not diminished by prior TLR ligand exposure. Furthermore, the production of interferon-beta (IFN-beta) following stimulation of TLR3 or -4, which is MyD88-independent, was increased by prior activation of TLR4, -5, -7 or -9. In contrast, TLR9 ligand-induced IFN-beta production, which is MyD88-dependent, was tolerized by prior TLR stimulation. These results are consistent with differential regulation of MyD88-dependent and MyD88-independent cytokine production following serial activation of TLRs.

The TLR4 agonist, monophosphoryl lipid A, attenuates the cytokine storm associated with respiratory syncytial virus vaccine-enhanced disease

Formalin-inactivated respiratory syncytial virus vaccine (FI-RSV) induces a poorly understood immunopathological response that leads to disease enhancement upon RSV infection of vaccinees. In the cotton rat model, inclusion of monophosphoryl lipid A (MPL) in the FI-RSV formulation was found to mitigate the lung pathology associated with vaccine-enhanced disease. Here we report that the protective effect of MPL on FI-RSV vaccine-enhanced disease is associated with a dramatic reduction in levels of Th1- and Th2-type cytokines and chemokines normally elicited in response to RSV challenge. Our data illustrate the complexity of proinflammatory response elicited by FI-RSV vaccination and RSV infection and the potential importance of MPL in modifying this response.

A potent adjuvant monophosphoryl lipid A triggers various immune responses, but not secretion of IL-1beta or activation of caspase-1

Lipid A, the membrane anchor portion of LPS, is responsible for the endotoxin activity of LPS and induces many inflammatory responses in macrophages. Monophosphoryl lipid A (MPL), a lipid A derivative lacking a phosphate residue, induces potent immune responses with low toxicity. To elucidate the mechanism underlying the low toxicity of MPL, we examined the effects of MPL on the secretion of proinflammatory cytokines by mouse peritoneal macrophages, a murine macrophage-like cell line (RAW 264.7), and a human macrophage-like cell line (THP-1). MPL enhanced the secretion of TNF-alpha, but not that of IL-1beta, whereas Escherichia coli-type lipid A (natural source-derived and chemically synthesized lipid A) enhanced the secretion of both cytokines. Although MPL enhanced the levels of IL-1beta mRNA and IL-1beta precursor protein to levels similar to those induced by lipid A, IL-1beta precursor processing in MPL-treated cells was much lower than that in E. coli-type lipid A-treated ones. Moreover, MPL, unlike E. coli-type lipid A, failed to induce activation of caspase-1, which catalyzes IL-1beta precursor processing. These results suggest that an immune response without activation of caspase-1 or secretion of IL-1beta results in the low toxicity of this adjuvant.

Brucella spp. lumazine synthase: a novel adjuvant and antigen delivery system to effectively induce oral immunity

Brucella lumazine synthase (BLS) has been previously used with success as a delivery system for systemic immunization against murine cysticercosis. We herein determined the usefulness of BLS as a new antigen-delivery system and mucosal-adjuvant using KETc1, one of the peptides of the anti-cysticercosis vaccine. A protection of up to 98% was induced when KETc1 was used as a chimera fused to BLS. Used as adjuvant of KETc1, BLS also induced a high level of protection (79%), which did not significantly differ from that induced by the cholera toxin (74%). KETc1 and BLS administered separately also reduced the parasite load. KETc1 administered orally as a chimera, and to a lesser extent with BLS as adjuvant, elicited IgG and IgA specific antibodies, which were detectable both in fecal extracts and in sera, and increased B and CD4 activated cells. BLS-KETc1 also increased the levels of transcription of TNF-alpha, IL-2 and IFNgamma in Peyer's patches, and in spleen, only increased TNF-alpha was observed. Overall, these results showed that BLS can be used as both an antigen-carrier and as an adjuvant in the design of new oral subunit vaccines.

Macrophages sequentially change their functional phenotype in response to changes in microenvironmental influences

Recent studies have described the development of distinct functional subsets of macrophages in association with cancer, autoimmune disease, and chronic infections. Based on the ability of Th1 vs Th2 cytokines to promote opposing activities in macrophages, it has been proposed that macrophages develop into either type 1 inflammatory or type 2 anti-inflammatory subsets. As an alternative to the concept of subset development, we propose that macrophages, in response to changes in their tissue environment, can reversibly and progressively change the pattern of functions that they express. As demonstrated herein, macrophages can reversibly shift their functional phenotype through a multitude of patterns in response to changes in cytokine environment. Macrophages display distinct functional patterns after treatment with IFN-gamma, IL-12, IL-4, or IL-10 and additional functional patterns are displayed depending on whether the cytokine is present alone or with other cytokines and whether the cytokines are added before or concomitantly with the activating stimulus (LPS). Sequential treatment of macrophages with multiple cytokines results in a progression through multiple functional phenotypes. This ability to adapt to changing cytokine environments has significant in vivo relevance, as evidenced by the demonstration that macrophage functional phenotypes established in vivo in aged or tumor-bearing mice can be altered by changing their microenvironment. A concept of functional adaptivity is proposed that has important implications for therapeutic targeting of macrophages in chronic diseases that result in the dominance of particular functional phenotypes of macrophages that play a significant role in disease pathology.

Mucosal adjuvants

Induction of immune responses following oral immunization is frequently dependent upon the co-administration of appropriate adjuvants that can initiate and support the transition from innate to adaptive immunity. The three bacterial products with the greatest potential to function as mucosal adjuvants are the ADP-ribosylating enterotoxins (cholera toxin and the heat-labile enterotoxin of Escherichia coli), synthetic oligodeoxynucleotides containing unmethylated CpG dinucleotides (CpG ODN), and monophosphoryl lipid A (MPL). The mechanism of adjuvanticity of the ADP-ribosylating enterotoxins is the subject of considerable debate. Our own view is that adjuvanticity is an outcome and not an event. It is likely that these molecules exert their adjuvant function by interacting with a variety of cell types, including epithelial cells, dendritic cells, macrophages, and possibly B- and T-lymphocytes. The adjuvant activities of CpG and MPL are due to several different effects they have on innate and adaptive immune responses and both MPL and CpG act through MyD88-dependent and -independent pathways. This presentation will summarize the probable mechanisms of action of these diverse mucosal adjuvants and discuss potential synergy between these molecules for use in conjunction with plant-derived vaccines.

Lipopolysaccharide and double-stranded RNA up-regulate toll-like receptor 2 independently of myeloid differentiation factor 88

Toll-like receptor 2 (TLR2) is a signaling receptor for a variety of microbial products, including bacterial lipoproteins and peptidoglycan, and is central in initiating immune responses toward Gram-positive bacteria, spirochetes, and mycobacteria. The mechanisms behind regulation of TLR2 protein expression are still not well understood. By using a newly developed monoclonal antibody against mouse TLR2, we detected TLR2 protein expression on macrophages, neutrophils, and dendritic cells. Endogenous macrophage TLR2 localized mostly to the cell membrane, with particular accumulation around phagosomes containing zymosan. Treatment of macrophages with the TLR2 antibody diminished cellular response to lipoproteins and down-regulated membrane TLR2. Marked up-regulation of surface TLR2 was observed on macrophages in response to whole bacteria, lipoproteins, lipopolysaccharide, poly(I-C) (double-stranded RNA), R848, and CpG DNA, and this up-regulation appeared to be a very sensitive marker for the presence of microbial products. Up-regulation of TLR2 in response to stimuli correlated with an increased response to secondary lipoprotein exposure following a low concentration of primary lipoprotein challenge. By comparison, exposure to a larger primary challenge induced a hyporeactive state. Most interestingly, lipopolysaccharide- and double-stranded RNA-induced up-regulation of surface TLR2 in macrophages was found to be MyD88-independent, whereas the up-regulation in response to lipoproteins, R848, and CpG DNA was absent in MyD88-deficient cells. We conclude that complex mechanisms regulate expression and signaling via TLR2. Up-regulation of TLR2 in the presence of low, yet clinically relevant amounts of microbial products may be an important mechanism by which the immune system boosts its response to a beginning infection.

N-linked glycosylation is required for c1 inhibitor-mediated protection from endotoxin shock in mice

C1 inhibitor (C1INH) prevents endotoxin shock in mice via a direct interaction with lipopolysaccharide (LPS). This interaction requires the heavily glycosylated amino-terminal domain of C1INH. C1INH in which N-linked carbohydrate was removed by using N-glycosidase F was markedly less effective in protecting mice from LPS-induced lethal septic shock. N-deglycosylated C1INH also failed to suppress fluorescein isothiocyanate (FITC)-LPS binding to and LPS-induced tumor necrosis factor alpha mRNA expression by the murine macrophage-like cell line, RAW 264.7, and cells in human whole blood. In an enzyme linked immunosorbent assay, the N-deglycosylated C1INH bound to LPS very poorly. In addition, C1INH was shown to bind to diphosphoryl lipid A (dLPA) but only weakly to monophosphoryl lipid A (mLPA). As with intact LPS, binding of N-deglycosylated C1INH to dLPA and mLPA was diminished in comparison with the native protein. Removal of O-linked carbohydrate had no effect on any of these activities. Neither detoxified LPS, dLPA, nor mLPA had any effect on the rate or extent of C1INH complex formation with C1s or on cleavage of the reactive center loop by trypsin. These data demonstrate that N-linked glycosylation of C1INH is essential to mediate its interaction with the LPA moiety of LPS and to protect mice from endotoxin shock.

Macrophages exposed continuously to lipopolysaccharide and other agonists that act via toll-like receptors exhibit a sustained and additive activation state

BACKGROUND

Macrophages sense microorganisms through activation of members of the Toll-like receptor family, which initiate signals linked to transcription of many inflammation associated genes. In this paper we examine whether the signal from Toll-like receptors [TLRs] is sustained for as long as the ligand is present, and whether responses to different TLR agonists are additive.

RESULTS

RAW264 macrophage cells were doubly-transfected with reporter genes in which the IL-12p40, ELAM or IL-6 promoter controls firefly luciferase, and the human IL-1beta promoter drives renilla luciferase. The resultant stable lines provide robust assays of macrophage activation by TLR stimuli including LPS [TLR4], lipopeptide [TLR2], and bacterial DNA [TLR9], with each promoter demonstrating its own intrinsic characteristics. With each of the promoters, luciferase activity was induced over an 8 hr period, and thereafter reached a new steady state. Elevated expression required the continued presence of agonist. Sustained responses to different classes of agonist were perfectly additive. This pattern was confirmed by measuring inducible cytokine production in the same cells. While homodimerization of TLR4 mediates responses to LPS, TLR2 appears to require heterodimerization with another receptor such as TLR6. Transient expression of constitutively active forms of TLR4 or TLR2 plus TLR6 stimulated IL-12 promoter activity. The effect of LPS, a TLR4 agonist, was additive with that of TLR2/6 but not TLR4, whilst that of lipopeptide, a TLR2 agonist, was additive with TLR4 but not TLR2/6. Actions of bacterial DNA were additive with either TLR4 or TLR2/6.

CONCLUSIONS

These findings indicate that maximal activation by any one TLR pathway does not preclude further activation by another, suggesting that common downstream regulatory components are not limiting. Upon exposure to a TLR agonist, macrophages enter a state of sustained activation in which they continuously sense the presence of a microbial challenge.

Multiple mechanisms of reduced major histocompatibility complex class II expression in endotoxin tolerance

Patients after polytrauma, burns, or septic shock frequently develop a life-threatening immunodeficiency. This state is associated with specific functional alterations of monocytic cells. We previously proposed endotoxin tolerance, the monocyte state after acute response to lipopolysaccharide, as a respective model system. One major feature in both the clinical situation and the in vitro model is the dramatic down-regulation of monocyte major histocompatibility complex (MHC) class II surface expression, which is associated with impaired antigen presentation capacity. This study focused on the mechanisms behind reduced MHC class II expression in endotoxin tolerance. Endotoxin priming provoked a decrease of monocyte intracellular MHC class II. It also led to a reduced expression of the chaperonic invariant chain and to an inhibited synthesis of the major lysosomal enzyme for final cleavage of the invariant chain going along with a relative accumulation of p10. The expression of HLA-DM necessary for loading MHC class II with antigenic peptide was also decreased. Additionally, reduced export of MHC class II alphabeta complexes to the cell surface was observed. The down-regulation of HLA-DR, invariant chain, and HLA-DM was regulated at the mRNA level and may be the consequence of reduced class II transactivator expression observed in this study. The simultaneous interference at different regulatory levels may explain the uniquely strong and long lasting MHC class II down-modulating effect of endotoxin priming compared with transforming growth factor-beta and interleukin-10. These results not only contribute to a better understanding of experimental endotoxin tolerance but may also give rise to new therapeutics for temporary immunodeficiency and, conversely, for MHC class II-dependent diseases such as autoimmunity and transplant rejection.

Role of innate immune factors in the adjuvant activity of monophosphoryl lipid A

Monophosphoryl lipid A (MPL) is a nontoxic derivative of lipopolysaccharide (LPS) that exhibits adjuvant properties similar to those of the parent LPS molecule. However, the mechanism by which MPL initiates its immunostimulatory properties remains unclear. Due to the involvement of Toll-like receptors in recognizing and transducing intracellular signals in response to LPS, the aim of the present study was to determine the ability of MPL to utilize the Toll-like receptor 2 (TLR2) and TLR4. We provide evidence that MPL differentially utilizes TLR2 and TLR4 for the induction of tumor necrosis factor alpha, interleukin 10 (IL-10), and IL-12 by purified human monocytes as well as by human peripheral blood mononuclear cells. Assessment of NF-kappa B activity demonstrated that MPL utilized TLR2 and especially TLR4 for the activation of NF-kappa B p65 by human monocytes. In addition, stimulation of human monocytes by MPL led to an up-regulation of the costimulatory molecules CD80 and CD86, an effect that could be reduced by pretreatment of cells with a monoclonal antibody to TLR2 or TLR4. Analysis of MPL-induced activation of the extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinases revealed that MPL utilized both TLR2 and TLR4 for the phosphorylation of ERK1/2, while TLR4 was the predominant receptor involved in the ability of MPL to phosphorylate p38. Moreover, using selective inhibitors for MAP kinase kinase (PD98059) and p38 (SB203580), we show that ERK1/2 exhibited differential effects on production of TNF-alpha and IL-12 p40 by human monocytes, whereas MPL-induced activation of p38 appeared to be predominantly involved in production of IL-10 and IL-12 p40 by MPL-stimulated monocytes. Taken together, these findings aid in understanding the cellular mechanisms by which MPL induces host cell activation and subsequent adjuvant properties.